This invention relates to a method for recording a color image and, in particular, a method for recording a full-color image in a color reproduction apparatus using black ink.
A thermal color printing system records a color image by superimposing a plurality of color inks on a recording medium. This system uses a color sequential recording technique, according to which a first color separation image is printed on the recording medium with a yellow ink, a second color separation image on the resultant image with a magenta ink and subsequently a third color separation image on the resultant image with a cyan ink.
A full color image thus printed contains areas printed with a single color ink and areas in which two or three color inks are superimposed. Particularly, black is represented by superimposing all three color inks. Since, however, light-transparent inks are used for thermal printing, it is not possible to realize a perfect black.
In order to realize a perfect black, a full color printing system is considered which uses a black ink. This system permits a ready reproduction of a perfect black color. This system permits a ready reproduction of delicate color tones and thus a reproduction of shadow details.
Conventionally, a UCR (under color removal) method is known for a color printing technique using a separate black ink. According to this method, black areas that would result from the superimposition of the reference color inks is predicted, and the predicted areas are printed simply by black ink. As shown in FIG. 7, for example, with Ay representing a yellow portion, Am representing magenta portion, and Ac representing a cyan portion in the unit pixel, a pixel must be represented as black if these portions are equal in area to each other. For this reason, the portion indicated as Abk in FIG. 7 is printed with black ink. According to this method it is possible to use an inexpensive black ink for printing.
The use of the UCR method in a thermal printer, however, does not allow good transfer of a black ink to a recording medium. This is caused by the characteristic of a full color image as well as the principle of a thermal color printing.
In general, in a full color image a black portion is not isolated in a colorless background. Namely, the black portion is in intimate contact with other color portions. In order to print a colored image by the UCR method, three color inks are properly superimposed on portions other than black portions, and the black portions remain as yet unprinted. The blank ink is then transferred onto those portions of a recording medium which have been left as unprinted.
A color image with the three-colors of ink is formed before the transfer of the black ink, thus resulting in a rise of a portion on which color inks are transferred relative to a portion where no ink transfer is made on the surface of a recording medium. Black ink is less easily transferred due to the creation of such a rise, meaning that a phenomenon occurs where the black ink remains unprinted on the portions intended to be printed.
In thermal printing, poor ink transfer, if any, adversely affects the quality of a printed image. The UCR method is not desirable for thermal color printing. For this reason, black ink is usually further printed in portions on which the three color inks have been already superimposed.
In the conventional four-color based full color thermal printing system, the image contrast is improved due to the addition of black ink. However, a printed color image becomes adversely darkened, resulting in the degradation of color balance.